System and method for liquefying natural gas employing turbo expander

ABSTRACT

An improved system and method for liquefying natural gas employing liquid nitrogen is disclosed. The improved system and method lowers the nitrogen consumption rate by using an expander, for example, a radial inflow turbo-expander, on the nitrogen side. This reduction in nitrogen consumption rate substantially reduces system operating costs.

RELATED APPLICATION INFORMATION

The present application claims priority under 35 U.S.C. Section 119(e)to U.S. Provisional Patent Application Ser. No. 62/021,602 filed Jul. 7,2014, the disclosure of which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for liquefyingnatural gas. More specifically, the present invention relates to systemsand methods for liquefying natural gas employing a cryogenic liquid suchas liquid nitrogen.

2. Description of the Prior Art and Related Information

Currently there are numerous ways to liquefy natural gas (NG). One ofthe ways is to use a cryogenic liquid such as nitrogen to liquefynatural gas (LNG). The present invention is directed to this type ofapproach. As an example, Cosmodyne LNG plant model PGL100 plant uses anopen nitrogen refrigeration cycle to liquefy natural gas (NG).

FIG. 1 shows a schematic sketch of such a prior art cryogenic naturalgas liquefier system 10. Nitrogen liquid from a storage tank 12 ispumped by a pump 14 into a coldbox 16 and is vaporized by the warmnatural gas. This heat exchange will result in liquefying the naturalgas.

The nitrogen refrigerant is supplied as a cryogenic liquid, liquidnitrogen (LN). It is boiled, superheated and eventually discharged asits refrigeration capacity is exhausted from cooling and liquefying thenatural gas. During the discharge process the nitrogen gas may passthrough a heater 24 and adsorption beds to form part of the NGpre-treatment system 18. The handling of the nitrogen gas exhaust mayvary depending on the intended use of the gas after it is discharged.

The discharged nitrogen gas is provided to an outlet vent 20 where itcan be vented to atmosphere or utilized in many different ways. Someuses include: reliquefaction in a nitrogen liquefier to be sent to a LNstorage tank; compression to be stored in standard nitrogen cylinders orused as an inert gas supply; or used as a regeneration gas for the NGpre-treatment system.

The coldbox will have a NG inlet, NG liquid outlets, a liquid nitrogeninlet and nitrogen gas outlets. The coldbox 16 typically comprises asteel frame which houses the heat exchangers (two sections; nitrogensuperheating region and reboiler/condenser), subcooling coils, piping,and pressure vessels (separators). The coldbox will be filled withinsulation material to minimize refrigeration loss to the ambientenvironment.

The NG is received from a supply 22 such as a pipeline. The NG will befirst directed to a pre-treatment system 18 to remove carbon dioxide(CO2), water (H2O), mercury (Hg), sulfur components (such as H2S), andother impurities. Depending on the feed gas composition a pre-treatmentsystem 18 can consist of just adsorption equipment or can include a feedgas inlet filter, separator, amine system, mercury guard bed, andadsorption equipment. The treated NG is then directed to the liquefiermodule including the coldbox 16. The pre-treatment of the natural gasupstream of the PGL100 system and the receipt of the product LNG fromthe coldbox at 26 may be conventional and are not described.

The advantage of this system is its simplicity, low capital investmentcost, and high reliability. There are very few components, making itrelatively cheap and quick to install. In addition, there are limitedmoving parts within the major equipment which avoids downtime forfrequent maintenance or repairs.

The drawback of this system is the high operating cost. The cryogenicliquid, usually liquid nitrogen, is expensive to source and supply.Additionally, the liquefier will require about 1.5 gallons of liquidnitrogen to produce 1 gallon of LNG. The actual ratio will depend on thesupply temperature and pressure of the liquid nitrogen.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides a natural gas liquefiersystem comprising of: a natural gas input coupled to a source of naturalgas; a liquid nitrogen input coupled to a source of liquid nitrogen; aliquefier module coupled to receive the natural gas and liquid nitrogenand liquefy the natural gas by boiling the liquid nitrogen; a turboexpander module coupled to the liquefier module to receive the boiledgaseous nitrogen, cool the gaseous nitrogen by expansion, andreintroduce the colder gaseous nitrogen into the liquefier module; and aliquefied natural gas output coupled to the liquefier module.

In another aspect the present invention provides a natural gas liquefiersystem, comprising a natural gas flow path including a natural gas inputand liquefied natural gas output, a separate nitrogen flow pathincluding a liquid nitrogen input and a gaseous nitrogen output, aliquefier module coupled to both the natural gas and nitrogen flow pathswherein the liquid nitrogen is brought into thermal contact with thenatural gas to liquefy the natural gas and boil the nitrogen, and aturbo expander coupled to the nitrogen flow path in a closed loop toreceive boiled gaseous nitrogen, cool the nitrogen gas through expansionand reintroduce the cooler nitrogen gas into the nitrogen flow path.

In another aspect the present invention provides a method of reducingliquid nitrogen usage in a natural gas liquefaction process in a systemhaving a natural gas liquefier module and a cryogenic turbo expander.The method comprises boiling liquid nitrogen in the natural gasliquefier module creating a nitrogen superheating region in theliquefier module and introducing two pressure levels of nitrogen coolingmedia in the nitrogen superheating region by using expansion of boilednitrogen in a turbo expander.

Further features and aspects of the invention are set out in thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a prior art cryogenic natural gasliquefier system.

FIG. 2 is a schematic drawing of an improved cryogenic natural gasliquefier system in accordance with the present invention.

FIG. 3 is a schematic drawing of a prior art coldbox employed in asystem such as that of FIG. 1.

FIG. 4 is a schematic drawing of a turbo expander module and coldbox inaccordance with the present invention.

FIG. 5 is a cross section of a suitable turbo expander employed in thesystem shown in FIG. 4.

FIG. 6 is a process flow diagram as implemented in the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved system and method forliquefying natural gas employing a cryogenic liquid such as liquidnitrogen (LN). The preferred implementation of the improved system andmethod lowers the nitrogen consumption rate by using an expander like aradial inflow turbo expander in the nitrogen side. This reduction innitrogen consumption rate is very important as it makes the describednatural gas liquefier competitive with other designs. As used herein theterm natural gas refers to both gas having naturally occurringhydrocarbons such as methane and ethane as well as various impuritiesand also the treated form of natural gas having some or all of theimpurities removed.

FIG. 2 shows a schematic sketch of an improved cryogenic natural gasliquefier system 100 improved in accordance with the present inventionas discussed in detail below. Several of the components illustrated maybe conventional in nature and will not be described in detail. Theoverall system has three main equipment systems, a LN pump module 110, aliquefier module or coldbox 120 and the turbo expander module 150. TheLN storage 122 and feed to the LN pump module may be conventional anddetails are not included in the present description. The pre-treatmentof the natural gas upstream of the system 100 and the receipt of theproduct LNG from the coldbox 120 at 124 are also conventional and notdescribed.

The coldbox 120 will have liquid and gas inlets and outlets for bothnatural gas and nitrogen shown in more detail in FIG. 4. The coldboxpreferably comprises a steel frame housing the heat exchanger(s) (twosections; nitrogen superheating and reboiler/condenser), recondensatecoils, piping, and pressure vessels (separator), as shown in FIG. 4. Theturbo expander module 150 will be comprised of a turbo expander,lubrication, cooling and seal gas system. The coldbox and cold sectionsof the turbo expander module 150 will be filled with insulation materialto minimize refrigeration losses.

The LN pump module 110 and turbo expander module 150 are controlled tooptimize nitrogen usage by the monitoring and control system 130 whichreceives inputs 132 from the coldbox 120 and turbo expanderinstrumentation as discussed below. Other conventional monitoring andcontrol functions are also provided which need not be described infurther detail.

The effluent nitrogen gas from the coldbox can be used for the adsorberbed regeneration by temperature swing adsorption regeneration, shown aspath 144 to the NG pre-treatment system 142. The nitrogen gas is thenprovided to an outlet vent 146 where it can be vented to atmosphere.Alternatively nitrogen gas from the coldbox can be used as feedstock toa nitrogen liquefier system 160 and sent to the LN storage tank 122forming a close loop nitrogen refrigeration system, thus furtherreducing operating cost.

On the natural gas (NG) side the NG will be first directed from aconventional supply 140 (such as a pipeline) to a pre-treatment system142 to remove carbon dioxide (CO2), water (H2O), mercury (Hg), sulfurcomponents (such as H2S), and other impurities. Depending on the feedgas composition the pre-treatment system can employ just adsorptionequipment 148 or can include a feed gas inlet filter, separator, aminesystem, mercury guard bed, and adsorption equipment. The treated NG isfed directly to the liquefier module including coldbox 120.

As noted above, the present invention provides a turbo expander module150. To better appreciate the advantages over the conventional approachwithout a turbo expander the conventional approach will first bedescribed in relation to FIG. 3. As shown in FIG. 3 the NG pressurecontroller 300 will provide a limiting function to a NG feed rate andwill be set consistently with the desired tank pressure.

The treated NG (Stream 310) enters the coldbox heat exchanger 301 whereit is precooled and withdrawn to a separator tank 302 (Stream 323),where the heavy hydrocarbons are removed in the liquid bottom stream ofthe separator. These are removed at this point of the process to preventthem from freezing inside the heat exchanger 301.

The gas stream from separator 302 will return to the heat exchanger 301(Stream 304) where it is cooled and partially condensed. The condensate,comprised predominantly of ethane and heavier hydrocarbons, is removedin separator 303, creating the capability to adjust the Wobbe number ofthe final LNG product. If the feed gas is rich in ethane, the bulk ofthe ethane can be removed from the process by the use of valve 305(Stream 321). For the expected feed gas composition, ethane rejectionwill not be necessary and the liquids removed in separator 303 will bere-injected into the product stream via LCV (Level Control Valve) 324.In this case valve 305 would remain closed.

The natural gas from separator 303 will flow (Stream 310) to the mainreboiler 314/condenser 312, a thermosyphon type heat exchanger, where NGis fully condensed and sub-cooled with a pool of boiling nitrogen. Thecondensed LNG drained from the reboiler vessel 314 (Stream 315) isblended with the sub-cooled ethane rich separator 303 bottoms (Stream316) to yield the final product stream (Stream 318). The final producttemperature will be in equilibrium with storage pressure.

On the nitrogen side, the nitrogen refrigerant is supplied as acryogenic liquid (LN) (Stream 320). LN from a storage tank ispressurized by pump modules 322 to provide positive control of the LNboiling temperature in the reboiler 314 (Stream 321). The gaseousnitrogen vapor (GN) created in the boiling process will flow out ofreboiler 314 and into heat exchanger 301 (Stream 315) to support NGcooling and liquid extraction in separator 303. In the process the GNvapor will become superheated. The temperature of the saturated GN vaporentering heat exchanger 301 must be controlled to achieve the desiredcooling of the NG feed gas and this is accomplished by monitoring thevapor temperature and controlling the LN level in reboiler vessel 314.The superheated gaseous nitrogen (GN) exits heat exchanger 301 forfurther utilization (Stream 317) which may be controlled by valve 325.To maintain the required temperature differential limits of heatexchanger 301, intermediate vents from heat exchanger 301 (Stream 326)may be provided when necessary. These vents can be routed and mergedtogether with final vent (Stream 317) if desired.

Next, the liquid nitrogen pump module 322 will be described. The purposeof the LN pump module is to deliver pressurized LN to the liquefier.Liquid nitrogen from storage will be pumped to elevated pressureconsistent with the condensing natural gas pressure and composition. Thepump module has two identical process pumps, as shown. One is used innormal operation and one is installed as a spare. Pressure control isachieved by PCV 325 located at the outlet of the coldbox.

The pump in operation is on an automatic speed control to maintain LNlevel in the boiler 314. This level is indicated by level indicator 330.Variable frequency drives (VFDs) located in the motor control center(MCC) room provide the speed loop control and the VFDs receive a remoteset point signal from the control system based on the LN level in boiler314. In this manner, the operator can change the liquid level inreboiler 314 by adjusting the pump speed set point. This capability isbuilt into the liquefier to control the heat transfer rate in heatexchanger 312. The pump module is equipped with a local panel where theoperator can switch the pumps on/off.

A preferred embodiment of the turbo expander module 150 as coupled tothe coldbox 120 in the improved liquefaction system of the presentinvention is shown in FIG. 4.

The treated NG (Stream 406) enters the coldbox heat exchanger 401 whereit is precooled and withdrawn to a separator tank 402 (Stream 423),where the heavy hydrocarbons are removed from the feed. These areremoved at this point in the process to prevent them from freezinginside the heat exchanger 401.

The gases from separator 402 return to the heat exchanger 401 (Stream304) where they are cooled and partially condensed. The condensate,comprised predominantly of ethane and heavier components is removed fromthe separator 303, creating the capability to adjust the Wobbe number ofthe final LNG product. If the feed gas is rich in ethane, the bulk ofthe ethane can be removed from the process by the use of valve 405(Stream 421). For the expected feed gas composition, ethane rejectionwill not be necessary and the liquids removed in separator 403 will bere-injected into the product stream via LCV (Level Control Valve) 424.In this case valve 405 would remain closed.

The natural gas from separator 403 will flow (Stream 410) to the mainreboiler (414)/condenser (412), a thermosyphon type heat exchanger,where NG is fully condensed and sub-cooled in a pool of boilingnitrogen. The condensed LNG drained from reboiler vessel 414 (Stream415) is blended with the sub-cooled ethane rich separator 403 bottoms(Stream 416) to yield the final product stream (Stream 418). The finalproduct temperature will be in equilibrium with storage pressure.

On the nitrogen side, the nitrogen refrigerant is supplied as acryogenic liquid (LN) (Stream 420) from a storage tank 122 (FIG. 2). LNfrom the storage tank is pressurized by pump modules 422 (Stream 421).The nitrogen vapor created during the boiling process in reboiler 414exits the reboiler and flows into heat exchanger 401 (Stream 417). Afterpartially heating, the nitrogen vapor is removed from heat exchanger 401and enters the turbo expander 430 (Stream 432). The radial inflow turboexpander may be conventional, such as a commercially available turboexpander like an ACD Corporation TC series turbo expander. A crosssection of a suitable expander is shown in FIG. 5, discussed below. Thetemperature of the saturated GN vapor entering heat exchanger 401 willbe a function of the LN flowrate and pressure to the turbo expanderinlet. The optimum operating point shall be a result of the followingcontrol signals and inputs: First, the expander inlet pressure sensor(signal 435) will signal and control the expander inlet guide vane (IGV)positioning (signal 434) to maintain expander inlet pressure; second,the liquid level sensor 450 will set the pump VFT controls (signal 132,FIG. 2) to regulate the flow rate of nitrogen. For a given natural gasfeed to the liquefier, there will be an optimum operating pointcontrolled as described above by LN pressure and flowrate.

The turbo expander 430 will lower the nitrogen pressure and temperaturethrough an isentropic expansion. The cold stream from the turbo expanderexhaust (Stream 436) will flow back into heat exchanger 401 to provideadditional refrigeration at the pinch point of the heat exchanger (401).This, in turn, will reduce the rate of liquid nitrogen required toliquefy the NG. The power generated by the turbo expander via isentropicexpansion can be utilized by a compressor mounted on the common shaft asthe expander to compress gas. As another option, the expander can beconnected to a generator instead and produce electrical power whenfeasible. As another option, the power generated by the expander may besimply dissipated using an oil or air brake.

The superheated GN exits heat exchanger 401 for further utilization orto be vented (Stream 438). The pressure of this exiting nitrogen gaswill be controlled by valve 440 and will be set based on its intendedfuture utilization.

The above description is particular to the thermosyphon type nitrogenboiling. It is selected to avoid freezing the methane. However, if theliquid nitrogen storage pressure is above 40 PSIG an alternative designwith a single heat exchanger shall be acceptable.

In such an alternate embodiment of the invention, a single heatexchanger (such as heat exchanger 401) may be sufficient for the desiredcooling of the natural gas. In such an embodiment the turbo expanderwould be coupled to the heat exchanger in a closed loop to enhanceefficiency, as in the embodiment shown in FIG. 4. In such an embodiment,LN from stream 420 and pumps 422 would be provided directly to heatexchanger 401 (similar to Stream 417 in FIG. 4). The output to theexpander and return of cooled GN (Stream 436) back to the exchanger willbe coupled at the top section of the exchanger vessel. LNG in turn wouldbe provided from heat exchanger 401 similarly to streams 415, 416 in theillustrated embodiment.

A suitable expander 430 employed in the system shown in FIG. 4 is shownin a sectional view in FIG. 5. The expander has an inlet 500 whichreceives GN stream 432 (FIG. 4). The inlet flow volume is controlled byinlet vane 502 which is mechanically opened or closed in response tocontrol signal 434 (FIG. 4) and the input nitrogen pressure is monitoredto provide feedback along line 435 (FIG. 4) to the control system 130(FIG. 2). The inlet control vane or other inlet volume control mechanismmay be configured as part of an input conduit to the expander instead ofpart of the expander, and the structure schematically shown is merelyillustrative in nature. The expander may operate using a conventionalexpander wheel 504 to provide low pressure, cooled GN flow (Stream 436)at expander outlet 506 (FIG. 4). As noted above, the power generated bythe turbo expander through isentropic expansion can be used to compressgas by a compressor mounted on the common shaft 510 as the expander. Thecompressor includes gas inlet 512 and outlet 514 and compressor wheel516 in a conventional configuration. Alternatively, shaft 510 mayconnect the expander to a generator to produce electrical power whenfeasible. Another option is to simply dissipate the energy using an oilor air brake. The illustrated turbo expander is one preferred expanderoption, but other expander designs may be employed to provide thedesired GN cooling.

Referring to FIG. 6, a simplified exemplary process flow implemented bycontrol system 130 is illustrated as it pertains to the modified controlof the present invention employing an expander in the control loop. Itwill be noted that a complete LNG plant control system may include manyother functions conventional in nature and not described. Initially, atarget reboiler LN level is set by the plant operator and is shown asinitial process step 600. The LN pump speed is then automaticallycontrolled by the process at step 602 to set and maintain LN level inthe reboiler 414 within the desired range. This level is maintainedunder the control of monitoring and control system 130 (FIG. 2). Next,at step 604 an initial expander inlet guide vane position for turboexpander 430 is set for an estimated desired minimal differentialtemperature (delta T) in the main heat exchanger. Next, at 606 thetemperature of the GN in heat exchanger 401 is monitored and provided tocontrol system 130 which also receives input from the expander IGVvalues and provides an adjusted signal to control LNG pump speed (602).Adjustment of the refrigeration capacity is performed at step 608. Bycontrolling the expander suction pressure (by adjusting IGV) and LNrates (by adjusting pump speed) in conjunction with monitoring GNtemperature in the heat exchanger (606), optimal cooling of the naturalgas may be achieved. This makes it possible to minimize the consumptionof LN while achieving the desired production rate of liquefied naturalgas. As a result, overall usage of LN is reduced, decreasing theoperation costs of the system.

Therefore, the present invention provides a number of features andaspects with attendant advantages. These include the following features,aspects and advantages. Enhancement of the natural gas open loopliquefaction process, where refrigeration is provided by boilingnitrogen by utilizing two pressure levels of nitrogen cooling media inthe nitrogen superheating region introduced by using expansion in thecryogenic turbo expander. Employing advanced, interactive, processcontrols to minimize nitrogen usage. Introducing a method for reclaimingwaste nitrogen back into the reliquefaction process.

It will be appreciated by those skilled in the art that the foregoing ismerely an illustration of the present invention in currently preferredimplementations. A wide variety of modifications to the illustratedembodiments are possible while remaining within the scope of the presentconvention. Therefore, the above description should not be viewed aslimiting but merely exemplary in nature.

What is claimed is:
 1. A natural gas liquefier system, comprising: anatural gas input coupled to a source of natural gas; a liquid nitrogeninput coupled to a source of liquid nitrogen; a liquefier module coupledto receive the natural gas and liquid nitrogen and liquefy the naturalgas by boiling the liquid nitrogen; a turbo expander module coupled tothe liquefier module to receive the boiled gaseous nitrogen, cool thegaseous nitrogen by expansion and reintroduce the colder gaseousnitrogen into the liquefier module; and a liquefied natural gas outputcoupled to the liquefier module.
 2. A natural gas liquefier system asset out in claim 1, wherein the turbo expander module comprises a turboexpander coupled to the liquefier module in a closed loop.
 3. A naturalgas liquefier system as set out in claim 1, wherein the liquefier modulecomprises at least one heat exchanger.
 4. A natural gas liquefier systemas set out in claim 3, wherein the at least one heat exchanger comprisesa first heat exchanger and a second heat exchanger, wherein the secondheat exchanger is coupled to the liquid nitrogen source and liquefiesthe natural gas by boiling the liquid nitrogen and outputs liquefiednatural gas to the liquefied natural gas output and outputs boiledgaseous nitrogen to the first heat exchanger, and wherein the turboexpander is coupled in a closed loop to the first heat exchanger.
 5. Anatural gas liquefier system as set out in claim 4, wherein the firstheat exchanger has first and second inputs for receiving gaseousnitrogen at a different temperature and different pressure from thesecond heat exchanger and expander, respectively.
 6. A natural gasliquefier system as set out in claim 1, further comprising a liquidnitrogen pump coupled between the liquid nitrogen input and theliquefier module.
 7. A natural gas liquefier system as set out in claim6, further comprising a monitoring and control system coupled to theliquid nitrogen pump, turbo expander module and liquefier module.
 8. Anatural gas liquefier system as set out in claim 7, wherein the turboexpander module comprises an input volume flow control mechanism.
 9. Anatural gas liquefier system as set out in claim 8, wherein themonitoring and control system controls the liquid nitrogen pump andturbo expander module input volume flow control mechanism.
 10. A naturalgas liquefier system as set out in claim 9, wherein the monitoring andcontrol system receives a liquid nitrogen level signal from theliquefier module and input pressure signal from the turbo expandermodule.
 11. A natural gas liquefier system as set out in claim 1,further comprising a nitrogen reclaiming system coupled to receive usednitrogen gas from the liquefier module, liquefy the nitrogen gas andreturn the liquid nitrogen to the liquid nitrogen source.
 12. A naturalgas liquefier system, comprising: a natural gas flow path including anatural gas input and liquefied natural gas outputs; a separate nitrogenflow path including a liquid nitrogen input and a gaseous nitrogenoutputs; a liquefier module coupled to both the natural gas and nitrogenflow paths wherein the liquid nitrogen is brought into thermal contactwith the natural gas to liquefy the natural gas and boil the nitrogen;and an expander coupled to the nitrogen flow path in a closed loop toreceive boiled gaseous nitrogen, cool the nitrogen gas through expansionand reintroduce the cooler nitrogen gas into the nitrogen flow path. 13.A natural gas liquefier system as set out in claim 12, wherein thenitrogen flow path includes a nitrogen reliquefaction system coupled toreceive nitrogen gas from the liquefier module, liquefy the nitrogen gasand return the liquid nitrogen to the nitrogen supply to the liquefiermodule.
 14. A natural gas liquefier system as set out in claim 12,wherein the liquefier module includes first and second heat exchangers,wherein the first heat exchanger is configured in the nitrogen flow pathin a nitrogen superheating region having boiled nitrogen in gaseousform, and wherein the expander is coupled to the first heat exchanger.15. A natural gas liquefier system as set out in claim 12, wherein thesecond heat exchanger comprises a reboiler which receives the liquidnitrogen and the natural gas, liquefies the natural gas and boils thenitrogen and provides the boiled nitrogen gas to the first heatexchanger at a first temperature and pressure.
 16. A natural gasliquefier system as set out in claim 15, wherein the expander providescooled nitrogen gas to the first heat exchanger at a second temperatureand pressure.
 17. A method of reducing liquid nitrogen usage in anatural gas liquefaction process in a system having a natural gasliquefier module and a cryogenic turbo expander, comprising: boilingliquid nitrogen in the natural gas liquefier module creating a nitrogensuperheating region in the liquefier module; and introducing twopressure levels of nitrogen cooling media in the nitrogen superheatingregion by using expansion of boiled nitrogen in the turbo expander. 18.A method as set out in claim 17, further comprising: monitoring nitrogenpressure input to the turbo expander; and controlling the flow of boilednitrogen into the turbo expander to control the pressure of boilednitrogen.
 19. A method as set out in claim 18, further comprising:monitoring liquid nitrogen level in the liquefier module; andcontrolling the pressure of liquid nitrogen supplied to the liquefiermodule.
 20. A method as set out in claim 17, further comprisingreclaiming and liquefying waste nitrogen and reintroducing the reclaimedliquid nitrogen into the liquefaction system.